This invention relates generally to a color television camera and to a system therein for controlling excessive incident light. More particularly, the invention relates to an excessive incident light control system in which an automatic compensation system maintains a constant output signal level from the camera tube of a color television camera. The system changes the operational point of the camera tube only when there is an excessive amount of incident light, with respect to the camera tube. The system is used up to a limit beyond which a television picture of good quality cannot be practically obtained. The iris aperture of the camera optical system is manually adjusted when a detecting means indicates that this usable limit has been. Thus, the camera pickup operates over a wide range of allowable incident light quantity .
In general, when an excessive amount of light is incident to the camera tube, or an image pickup tube, of a color television camera (hereinafter called simply "color camera") the picture quality of the television picture deteriorates. Accordingly, there have been various systems in the prior art for substantially controlling the amount of light incident to the camera tube of a color camera in order to pick up television pictures of good quality.
Representative examples of these control systems are: (1) the so-called auto-iris system in which an aperture-varying or iris mechanism provided within the optical system of the color camera is adjusted by an automatic control; (2) a system in which an iris mechanism within the optical system of the color camera is manually adjusted; and (3) the automatic gain control system (AGC system) in which, the beam current of the camera tube is left at a large value. The range of correspondence between the amount of incident light and the signal output of the camera tube (i.e., dynamic range) is large. The output signal of the camera tube is constant. These systems have been accompanied by the following problems.
The auto-iris system (1) has required a long response time since its iris mechanism is automatically controlled in a mechanical manner by a means, such as a servomotor. It is necessary to provide means for clipping signals, such as a nonlinear element outside of the automatic control loop. The price of the apparatus is very high.
While the above mentioned control system (2) is inexpensive and is therefore suitable for color cameras of a simple type, the continual manual adjustment of the iris mechanism is troublesome, and the control manipulation of the camera becomes complex.
In the AGC system (3), the uniformity in the resulting picture of the output modulated chrominance signal, of the camera tube, deteriorates in proportion to the beam current. Consequently, it has been possible to control excessive amounts of incident light of a certain magnitude in the case of a camera tube of the electromagnetic focussing-electromagnetic deflection type. However, the maximum usable beam current, in the case of a camera tube of the electrostatic focussing-electromagnetic deflection type, has been limited to approximately twice the normal beam current.
Accordingly, it is contemplated in the present invention to overcome the above described problems encountered in the prior art systems. The invention uses an automatic level control system (hereinafter referred to as ALC system), which has heretofore been considered to be inapplicable in a color camera. There are automatic control means whereby the output signal level becomes constant even for excessive amounts of incident light. Indicating means indicate the usable limit point of the ALC system. The iris aperture of the camera lens system can be manually adjusted upon detection of the usable limit point, as indicated by the indicating means.
The ALC system is one wherein the target voltage of the camera tube is varied and the sensitivity thereof is adjusted by an electrical circuit. The circuit causes the output signal level of the camera tube to be constant, irrespective of variation in the incident light quantity entering the camera tube. This ALC system is being used in monochrome cameras.
In a color camera, it is necessary to hold constant the target voltage of the camera tube for preventing black level clamping (set up) and shading and preventing variation of the gamma (.gamma.) characteristic of the camera tube output signal. For this reason, it has heretofore been considered impossible to apply the above mentioned ALC system, in which the target voltage is varied, to a color camera, and it has not been realized in practice.
We have found, however, that an ALC system can be applied in a color camera provided that the following three conditions are satisfied.
First, the target voltage of the camera must not be raised above a level (hereinafter referred to as the standard image pickup level) in which the sensitivity is such that the target voltage is raised so high the shading and temperature characteristic do not present problems in practice. That is, as the target voltage is raised and the sensitivity of the camera tube increases, shading is produced until the resulting television picture becomes practically unbearable. Accordingly, the target voltage is held constant for incident light quantities which are less than an excessive incident light quantity. The target voltage is decreased and the sensitivity is decreased by means of the ALC system only for excessive incident light quantity.
Second, the limiting range is at the point where a picture of good quality in practice cannot be obtained because of white balance deviation due to nonuniformity of the color signal components red (R), green (G), and blue (B). This limit is caused when the gamma characteristic, of the output signal of the camera tube, used in a simple type color camera, is not 1 (unity). A variation of set up is due to variation in shading in the range of practical use of the ALC system.
Third, means are provided so that the camera operator can judge the range of applicability of the ALC system. This means is necessary for the camera operator when the range, of practical use, of the ALC system is exceeded. The operator must detect when the range is exceeded and manually control the lens aperture of the camera optical system thereby the adjust the incident light quantity to a suitable magnitude.